bundle2: add a interrupt mechanism It is now possible to emit a single part in the middle of a payload production. This part will be processed with limitation (only access to a `ui` object). The goal is to let the server raise exception and output while a part is being processed. The source motivation is to transmit exception that occurs while generating a part. This change is was the motivation to bump the bundle2 format from HG2X to HG2Y. Somehow, the format bump made it into 3.2 without it. So this change go on stable. It is low risk as bundle2 is still disabled by default.